Conventionally, there has been known two types of developing processes for use in the electrophotographic image forming apparatus; a single-component developing method using only toner as a principal component of the developer and a two-component developing method using toner and carrier in combination as principal components of the developer.
The developing device of the single-component developing method has a toner transporting member which transports toner and a frictional charging member which contacts with a toner transporting surface of the toner transporting member. With this arrangement, the toner transported by the toner transporting member is brought into the contact region of the frictional charging member, where it is formed into a thinned layer of toner charged with a certain polarity. Since the single-component development system charges the toner through the contact with the frictional charging member, the developing device has a simple and compact structure and therefore may be constructed economically. Contrarily, the toner is subjected to a strong stress in the contact region of the frictional charging member, which can damage the toner and deteriorate its charging ability in a short period of time. Also, the toner can adhere to the toner transporting member and/or the frictional charging member due to the contact pressure, which reduces charging abilities of those members and, as a result, shortens the life of the developing device.
Typically, the developing device using the two-component developing method causes the toner and the carrier to be charged into opposite polarities through the frictional contacts thereof. Therefore, the stress exerted on the toner is less than that in the developing device of the single-component developing method. Also, the carrier has a larger surface area than the toner, which provides less contamination to the carrier by the possible adhesion of the toner. However, an amount of toner fixed on the surface of the carrier, called toner-spent, increases with a mixing time, which in turn reduces the charging ability against the toner and causes problems such as unwanted fog-like toner adhesion onto the resultant image and/or scattering of the insufficiently charged toner into the air. It may be thought that the life of the two-component developing device be extended by increasing an amount of carrier contained in the developing device. Disadvantageously, this results in a considerable increase in size of the developing device.
In order to solve the problems described for the two-component developing device, JP 59-100472 A discloses a developing device in which an increase of deteriorated carrier is restricted by supplying new carrier only or new carrier and toner in combination to the developer while discharging a part of the deteriorated developer, intermittently. This, indeed, extends the life of the developer without increasing the size of the developing device but, disadvantageously, needs a mechanism for collecting the discharged carrier and also uneconomically wastes a considerable amount of carrier, which can cause an environmental problem. Further, a considerable number of printings are required to establish a predetermined ratio between not-deteriorated and deteriorated carriers.
JP 09-269614 A discloses a carrier and an image forming method using the carrier, in which each particle of carrier has a core material and a resin layer covering the core, the layer being made of a matrix resin and electrically conductive small particles dispersed in the matrix resin. In use, even if a portion or portions of the surface of this carrier are chipped off by the contact with other particles such as carrier particles and toner particles and/or components such as roller and screw, the underlying resin particles are alternatively exposed at the surface of the carrier, so that they make contacts with the toner to provide the toner with a necessary electric charge. However, a thickness of a resin cover layer is limited and then, once the layer is consumed, the carrier reaches the end of its life.
JP 2003-215855 A discloses a two-component developer comprising a carrier and a toner which carries chargeable particles on the surface thereof and an image forming method using the developer. The charging particles function as polishing material for removing the toner-spent which is the adhesion of the toner onto the carrier and thereby extending the life of the carrier. JP 2003-215855 Also discloses that the charging particle polishes the surface of an electrostatic latent image bearing member in the region for cleaning the bearing surface of the electrostatic latent image bearing member. Disadvantageously, the charging particle tends to be electrically charged into a polarity which is different from the polarity provided to the toner. This results in that the charging particle is likely to adhere to a region outside the image forming region, i.e., non-image forming region, of the electrostatic latent image bearing member and then consumed in a short period of time. In particular, a production of a number of images each with smaller image portions such as text images wastes a great amount of charging particles to restrict the polishing and the resultant reproduction characteristics of the carrier.
JP 2006-308687 A discloses an image forming apparatus having a developing device with a magnetic roller and a developing roller. The magnetic roller supports a developing material including toner and carrier. The carrier is magnetically retained on the magnetic roller and electrically holds the toner which is selectively provided to the developing roller for the development of the electrostatic latent images on the electrostatic latent image bearing member into visualized images. In particular, the developer also includes the charging particles existing but not electrically or magnetically retained between toner and carrier particles to prevent the unwanted adhesion of the toner and thereby the generation of the toner-spent on the carrier. The charging particles, however, are contained only in the toner initially introduced into the developing device and also consumed gradually with the consumption of the toner, due to the electrical connection with the toner, and supplied through the developing roller to the non-image forming region of the electrostatic latent image bearing member. As a result, a mass production of the image with a smaller image region or black-to-white ratio such as text image causes a considerable amount of charging particles to be wasted and thereby fails to ensure a long term, stable electric charging of the toner.